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<pre><span class="sourceLineNo">001</span>/*<a name="line.1"></a>
<span class="sourceLineNo">002</span> * Copyright (C) 2007 The Guava Authors<a name="line.2"></a>
<span class="sourceLineNo">003</span> *<a name="line.3"></a>
<span class="sourceLineNo">004</span> * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except<a name="line.4"></a>
<span class="sourceLineNo">005</span> * in compliance with the License. You may obtain a copy of the License at<a name="line.5"></a>
<span class="sourceLineNo">006</span> *<a name="line.6"></a>
<span class="sourceLineNo">007</span> * http://www.apache.org/licenses/LICENSE-2.0<a name="line.7"></a>
<span class="sourceLineNo">008</span> *<a name="line.8"></a>
<span class="sourceLineNo">009</span> * Unless required by applicable law or agreed to in writing, software distributed under the License<a name="line.9"></a>
<span class="sourceLineNo">010</span> * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express<a name="line.10"></a>
<span class="sourceLineNo">011</span> * or implied. See the License for the specific language governing permissions and limitations under<a name="line.11"></a>
<span class="sourceLineNo">012</span> * the License.<a name="line.12"></a>
<span class="sourceLineNo">013</span> */<a name="line.13"></a>
<span class="sourceLineNo">014</span><a name="line.14"></a>
<span class="sourceLineNo">015</span>package com.google.common.collect;<a name="line.15"></a>
<span class="sourceLineNo">016</span><a name="line.16"></a>
<span class="sourceLineNo">017</span>import static com.google.common.base.Preconditions.checkNotNull;<a name="line.17"></a>
<span class="sourceLineNo">018</span><a name="line.18"></a>
<span class="sourceLineNo">019</span>import com.google.common.annotations.Beta;<a name="line.19"></a>
<span class="sourceLineNo">020</span>import com.google.common.annotations.GwtIncompatible;<a name="line.20"></a>
<span class="sourceLineNo">021</span>import com.google.common.base.Equivalence;<a name="line.21"></a>
<span class="sourceLineNo">022</span>import com.google.common.base.Function;<a name="line.22"></a>
<span class="sourceLineNo">023</span>import com.google.common.collect.MapMakerInternalMap.ReferenceEntry;<a name="line.23"></a>
<span class="sourceLineNo">024</span><a name="line.24"></a>
<span class="sourceLineNo">025</span>import java.util.concurrent.ConcurrentMap;<a name="line.25"></a>
<span class="sourceLineNo">026</span><a name="line.26"></a>
<span class="sourceLineNo">027</span>/**<a name="line.27"></a>
<span class="sourceLineNo">028</span> * Contains static methods pertaining to instances of {@link Interner}.<a name="line.28"></a>
<span class="sourceLineNo">029</span> *<a name="line.29"></a>
<span class="sourceLineNo">030</span> * @author Kevin Bourrillion<a name="line.30"></a>
<span class="sourceLineNo">031</span> * @since 3.0<a name="line.31"></a>
<span class="sourceLineNo">032</span> */<a name="line.32"></a>
<span class="sourceLineNo">033</span>@Beta<a name="line.33"></a>
<span class="sourceLineNo">034</span>public final class Interners {<a name="line.34"></a>
<span class="sourceLineNo">035</span>  private Interners() {}<a name="line.35"></a>
<span class="sourceLineNo">036</span><a name="line.36"></a>
<span class="sourceLineNo">037</span>  /**<a name="line.37"></a>
<span class="sourceLineNo">038</span>   * Returns a new thread-safe interner which retains a strong reference to each instance it has<a name="line.38"></a>
<span class="sourceLineNo">039</span>   * interned, thus preventing these instances from being garbage-collected. If this retention is<a name="line.39"></a>
<span class="sourceLineNo">040</span>   * acceptable, this implementation may perform better than {@link #newWeakInterner}. Note that<a name="line.40"></a>
<span class="sourceLineNo">041</span>   * unlike {@link String#intern}, using this interner does not consume memory in the permanent<a name="line.41"></a>
<span class="sourceLineNo">042</span>   * generation.<a name="line.42"></a>
<span class="sourceLineNo">043</span>   */<a name="line.43"></a>
<span class="sourceLineNo">044</span>  public static &lt;E&gt; Interner&lt;E&gt; newStrongInterner() {<a name="line.44"></a>
<span class="sourceLineNo">045</span>    final ConcurrentMap&lt;E, E&gt; map = new MapMaker().makeMap();<a name="line.45"></a>
<span class="sourceLineNo">046</span>    return new Interner&lt;E&gt;() {<a name="line.46"></a>
<span class="sourceLineNo">047</span>      @Override public E intern(E sample) {<a name="line.47"></a>
<span class="sourceLineNo">048</span>        E canonical = map.putIfAbsent(checkNotNull(sample), sample);<a name="line.48"></a>
<span class="sourceLineNo">049</span>        return (canonical == null) ? sample : canonical;<a name="line.49"></a>
<span class="sourceLineNo">050</span>      }<a name="line.50"></a>
<span class="sourceLineNo">051</span>    };<a name="line.51"></a>
<span class="sourceLineNo">052</span>  }<a name="line.52"></a>
<span class="sourceLineNo">053</span><a name="line.53"></a>
<span class="sourceLineNo">054</span>  /**<a name="line.54"></a>
<span class="sourceLineNo">055</span>   * Returns a new thread-safe interner which retains a weak reference to each instance it has<a name="line.55"></a>
<span class="sourceLineNo">056</span>   * interned, and so does not prevent these instances from being garbage-collected. This most<a name="line.56"></a>
<span class="sourceLineNo">057</span>   * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative<a name="line.57"></a>
<span class="sourceLineNo">058</span>   * when the memory usage of that implementation is unacceptable. Note that unlike {@link<a name="line.58"></a>
<span class="sourceLineNo">059</span>   * String#intern}, using this interner does not consume memory in the permanent generation.<a name="line.59"></a>
<span class="sourceLineNo">060</span>   */<a name="line.60"></a>
<span class="sourceLineNo">061</span>  @GwtIncompatible("java.lang.ref.WeakReference")<a name="line.61"></a>
<span class="sourceLineNo">062</span>  public static &lt;E&gt; Interner&lt;E&gt; newWeakInterner() {<a name="line.62"></a>
<span class="sourceLineNo">063</span>    return new WeakInterner&lt;E&gt;();<a name="line.63"></a>
<span class="sourceLineNo">064</span>  }<a name="line.64"></a>
<span class="sourceLineNo">065</span><a name="line.65"></a>
<span class="sourceLineNo">066</span>  private static class WeakInterner&lt;E&gt; implements Interner&lt;E&gt; {<a name="line.66"></a>
<span class="sourceLineNo">067</span>    // MapMaker is our friend, we know about this type<a name="line.67"></a>
<span class="sourceLineNo">068</span>    private final MapMakerInternalMap&lt;E, Dummy&gt; map = new MapMaker()<a name="line.68"></a>
<span class="sourceLineNo">069</span>          .weakKeys()<a name="line.69"></a>
<span class="sourceLineNo">070</span>          .keyEquivalence(Equivalence.equals())<a name="line.70"></a>
<span class="sourceLineNo">071</span>          .makeCustomMap();<a name="line.71"></a>
<span class="sourceLineNo">072</span><a name="line.72"></a>
<span class="sourceLineNo">073</span>    @Override public E intern(E sample) {<a name="line.73"></a>
<span class="sourceLineNo">074</span>      while (true) {<a name="line.74"></a>
<span class="sourceLineNo">075</span>        // trying to read the canonical...<a name="line.75"></a>
<span class="sourceLineNo">076</span>        ReferenceEntry&lt;E, Dummy&gt; entry = map.getEntry(sample);<a name="line.76"></a>
<span class="sourceLineNo">077</span>        if (entry != null) {<a name="line.77"></a>
<span class="sourceLineNo">078</span>          E canonical = entry.getKey();<a name="line.78"></a>
<span class="sourceLineNo">079</span>          if (canonical != null) { // only matters if weak/soft keys are used<a name="line.79"></a>
<span class="sourceLineNo">080</span>            return canonical;<a name="line.80"></a>
<span class="sourceLineNo">081</span>          }<a name="line.81"></a>
<span class="sourceLineNo">082</span>        }<a name="line.82"></a>
<span class="sourceLineNo">083</span><a name="line.83"></a>
<span class="sourceLineNo">084</span>        // didn't see it, trying to put it instead...<a name="line.84"></a>
<span class="sourceLineNo">085</span>        Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);<a name="line.85"></a>
<span class="sourceLineNo">086</span>        if (sneaky == null) {<a name="line.86"></a>
<span class="sourceLineNo">087</span>          return sample;<a name="line.87"></a>
<span class="sourceLineNo">088</span>        } else {<a name="line.88"></a>
<span class="sourceLineNo">089</span>          /* Someone beat us to it! Trying again...<a name="line.89"></a>
<span class="sourceLineNo">090</span>           *<a name="line.90"></a>
<span class="sourceLineNo">091</span>           * Technically this loop not guaranteed to terminate, so theoretically (extremely<a name="line.91"></a>
<span class="sourceLineNo">092</span>           * unlikely) this thread might starve, but even then, there is always going to be another<a name="line.92"></a>
<span class="sourceLineNo">093</span>           * thread doing progress here.<a name="line.93"></a>
<span class="sourceLineNo">094</span>           */<a name="line.94"></a>
<span class="sourceLineNo">095</span>        }<a name="line.95"></a>
<span class="sourceLineNo">096</span>      }<a name="line.96"></a>
<span class="sourceLineNo">097</span>    }<a name="line.97"></a>
<span class="sourceLineNo">098</span><a name="line.98"></a>
<span class="sourceLineNo">099</span>    private enum Dummy { VALUE }<a name="line.99"></a>
<span class="sourceLineNo">100</span>  }<a name="line.100"></a>
<span class="sourceLineNo">101</span><a name="line.101"></a>
<span class="sourceLineNo">102</span>  /**<a name="line.102"></a>
<span class="sourceLineNo">103</span>   * Returns a function that delegates to the {@link Interner#intern} method of the given interner.<a name="line.103"></a>
<span class="sourceLineNo">104</span>   *<a name="line.104"></a>
<span class="sourceLineNo">105</span>   * @since 8.0<a name="line.105"></a>
<span class="sourceLineNo">106</span>   */<a name="line.106"></a>
<span class="sourceLineNo">107</span>  public static &lt;E&gt; Function&lt;E, E&gt; asFunction(Interner&lt;E&gt; interner) {<a name="line.107"></a>
<span class="sourceLineNo">108</span>    return new InternerFunction&lt;E&gt;(checkNotNull(interner));<a name="line.108"></a>
<span class="sourceLineNo">109</span>  }<a name="line.109"></a>
<span class="sourceLineNo">110</span><a name="line.110"></a>
<span class="sourceLineNo">111</span>  private static class InternerFunction&lt;E&gt; implements Function&lt;E, E&gt; {<a name="line.111"></a>
<span class="sourceLineNo">112</span><a name="line.112"></a>
<span class="sourceLineNo">113</span>    private final Interner&lt;E&gt; interner;<a name="line.113"></a>
<span class="sourceLineNo">114</span><a name="line.114"></a>
<span class="sourceLineNo">115</span>    public InternerFunction(Interner&lt;E&gt; interner) {<a name="line.115"></a>
<span class="sourceLineNo">116</span>      this.interner = interner;<a name="line.116"></a>
<span class="sourceLineNo">117</span>    }<a name="line.117"></a>
<span class="sourceLineNo">118</span><a name="line.118"></a>
<span class="sourceLineNo">119</span>    @Override public E apply(E input) {<a name="line.119"></a>
<span class="sourceLineNo">120</span>      return interner.intern(input);<a name="line.120"></a>
<span class="sourceLineNo">121</span>    }<a name="line.121"></a>
<span class="sourceLineNo">122</span><a name="line.122"></a>
<span class="sourceLineNo">123</span>    @Override public int hashCode() {<a name="line.123"></a>
<span class="sourceLineNo">124</span>      return interner.hashCode();<a name="line.124"></a>
<span class="sourceLineNo">125</span>    }<a name="line.125"></a>
<span class="sourceLineNo">126</span><a name="line.126"></a>
<span class="sourceLineNo">127</span>    @Override public boolean equals(Object other) {<a name="line.127"></a>
<span class="sourceLineNo">128</span>      if (other instanceof InternerFunction) {<a name="line.128"></a>
<span class="sourceLineNo">129</span>        InternerFunction&lt;?&gt; that = (InternerFunction&lt;?&gt;) other;<a name="line.129"></a>
<span class="sourceLineNo">130</span>        return interner.equals(that.interner);<a name="line.130"></a>
<span class="sourceLineNo">131</span>      }<a name="line.131"></a>
<span class="sourceLineNo">132</span><a name="line.132"></a>
<span class="sourceLineNo">133</span>      return false;<a name="line.133"></a>
<span class="sourceLineNo">134</span>    }<a name="line.134"></a>
<span class="sourceLineNo">135</span>  }<a name="line.135"></a>
<span class="sourceLineNo">136</span>}<a name="line.136"></a>




























































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